It’s a full 40 pages PDF document about language specifications, so it’s quite heavy but just in the very first page there’s a interesting information about the actual status of the language specifications:

This document reflects the understanding of the language as of October 2007. TG1 is no longer accepting proposals and has started writing the language specification. The ES4 reference implementation is undergoing stabilization and is already passing large parts of its test suite, and this document describes the final language in most respects. That said, TG1 maintains a bug tracking system at http://bugs.ecmascript.org/, and ES4 is subject to further refinement as known and new bugs in the design are resolved.

latter on page 4:

The current Task Group 1 of Ecma Technical Committee 39 (Ecma TC39-TG1, hereinafter TG1) has met
since the fall of 2005, basing their work on ES3, the interim ES4 report, and experiences with Adobe’s and
thMicrosoft’s implementations. TG1 expects to complete the 4 Edition Specification in the fall of 2008.

These lecture notes are mainly excerpts from the overview document, showing a lot of interesting code examples and is much more reader friendly.

I found a lot of the new stuff very interesting and my personal tops are:

array type

operator overloading

generators

Vector class

destructuring assignment

triple-quoted strings

lexical block scoping

and other minors features too (mainly mutuated from python) like:

string indexing

slicing syntax

(the Mock’s document hasn’t anchors so i cannot link the above list, however just copy/find in page).

The most waited by many people is typed arrays, because let’s say it… 9 out of 10 times you use arrays all the elements are of the same type.
With typed arrays the virtual machine doesn’t need to check every element and everything should gain a lot of speed and make as4 a lot faster than as3 in particles, swarms, lists and bitmapData’s operations.